The present invention relates to a liquid delivery system and, more particularly, to a liquid delivery system that is capable of repeatedly delivering a small amount of precisely metered liquid.
Devices for delivering precisely measured or metered minute amounts of liquids have a wide range of applications, ranging from ink jet printing to drug administration and combinatorial chemistry (See, e.g., Demers et al., U.S. Pat. No. 6,033,544). In ink jet printing alone, it is estimated that nearly 120 million ink jet cartridges were sold in 1998 in the United States, and the size of the worldwide market is predicted to more than double by 2003.
Presently, most commercially-available ink jet print heads employ either a piezoelectric crystal, to which a high voltage is applied to cause the crystal to deform and apply pressure on an ink reservoir, or electrothermal drop ejection, in which the ink is rapidly heated to a high temperature to evaporate a small quantity of ink. The vaporized ink forms a bubble that creates a pressure wave within the ink reservoir that forces drops of ink to be ejected. While these technologies have generally functioned well, they also have shortcomings that limit their desirability. For example, piezoelectric printing mechanisms require highly complex circuitry and bulky crystal arrays that make batch fabrication difficult. On the other hand, thermal ink jets require greater power consumption to heat the ink, and only special aqueous inks may be used. Both piezoelectric and thermal technologies facilitate drop delivery at a frequency of about 12 kHz and a volume of generally greater than 3 pL, which puts a limit on improvements to print speed and print resolution.
Thus, there remains a need for an improved liquid delivery system which may be advantageously used in a wide variety of applications, including ink jet printing.
It is a further object to provide such a liquid delivery system that is subject to batch fabrication techniques.
An additional object of the present invention is to provide a liquid delivery system that is capable of delivering liquid at a greater frequency and in smaller quantities than currently available technologies.
These objects, as well as others which will become apparent upon reference to the following detailed description and accompanying drawings, are provided by a liquid delivery system that is fabricated on a substrate that comprises at least one liquid source and a chamber including a nozzle with an outlet through which liquid is expelled from the system. A microchannel connects the liquid source to the chamber, and a pump transports liquid from a liquid source to the chamber. A moveable member is provided for applying positive pressure to the liquid in the chamber, so as to force a metered amount of liquid through the outlet of the nozzle. A reciprocating mass is operatively connected to the moveable member to move it back and forth.
In one embodiment, the moveable member is a generally 3-sided, U-shaped structure having spaced-apart sides, each of which is sized and shaped to fit through the outlet of the nozzle. A third side connects the spaced-apart sides to each other, and the entire structure is reciprocable between a first position inside the chamber and a second position outside the nozzle. As the structure moves from the inside of the nozzle through the outlet, a liquid film forms on the open area defined by the structure, and liquid in excess of that needed to form the film forms a drop.
In a second embodiment, the moveable member comprises a 4-sided, generally rectangularly-shaped structure with opposed open sides. Two of the opposing sides are sized and shaped to fit through the outlet of the nozzle, while the other two opposing sides connect the first two opposing sides. The structure is reciprocable by the moveable member between a first position inside the chamber and a second position outside the chamber. As the structure moves through the outlet of the nozzle, a liquid film forms across the open sides and the liquid in excess of that required to form the film forms a drop.
In a third embodiment, the liquid delivery system includes a valve that substantially seals the chamber from the microchannel, and the moveable member comprises a generally flat plate. The bottom side of the plate is submerged beneath the surface of the liquid in the chamber, and the flat plate is reciprocable between a first position in which no force is exerted by the plate on the surface of the liquid to a second position in which a force is exerted on the surface of the liquid. The surface tension of the liquid with respect to the top edge of the plate is such that, as the plate exerts pressure on the liquid, liquid does not move across the top edge of the plate onto the top side of the plate, and liquid is forced out of the chamber through the nozzle.
In a fourth embodiment, the moveable member includes a portion that seals the microchannel from the chamber as the moveable member moves from a first position to a second position where liquid is forced through the outlet of the nozzle.
In a fifth embodiment, the liquid delivery system includes at least one valve to seal the chamber from the microchannel, and the moveable member comprises a tapered plunger, the outlet of the chamber having an outlet of a shape complimentary to the plunger.
In a sixth embodiment, the liquid delivery system includes a nozzle that has edges which are engaged by the moveable member. The moveable member comprises a flat plate sized so that as the plate passes through the chamber, the plate engages the edges of the nozzle to seal the nozzle from the microchannel. Flexing of the flat plate after it engages the edges of the nozzle causes liquid to be expelled through the outlet of the nozzle.
In a seventh embodiment, the moveable member of the liquid delivery system is an elongated member having an axis along its length and a first end that secured to a support interior of the chamber and a second free end. The elongated member is moveable between a first position spaced-apart from the nozzle to a second position closely overlying the nozzle so that a metered amounted of liquid is forced through the nozzle as the elongated member moves from its first position to its second position. The reciprocating mass may be operatively connected to the free end of the elongated member so as to apply a force either perpendicular to the axis or in line with the axis.
Each of the embodiments is on a scale such that surface tension dominates over gravitational forces. This allows the liquid delivery system to be oriented in any reasonable direction in relation to the local gravitational field.
It is contemplated that the liquid delivery system of the present invention will be fabricated by micromachining using any of the well-known processes. As disclosed, the liquid delivery system uses micro-electro-mechanical structures, popularly known as MEMS for actuating the moveable member.
The devices employ microchannels of capillary dimensions, that is they favor capillary flow of liquid, with a barrier to fluid flow out of the system due to the formation by the liquid of an energy-minimizing surface such as a meniscus.